1. Field of the Invention
The present invention relates to detection circuits and more particularly to monitoring an AC waveform for its DC content.
2. Description of the Prior Art
Conventional AC generators and transformer coupled AC power converters provide AC waveforms which are free of steady-state DC components by virtue of the isolation provided by their electromagnetic circuits. For generators, the field is isolated from the armature. For transformer coupled AC power converters the output primary windings are isolated from the output secondary windings.
Some power conversion topologies do not provide for direct isolation, however. A DC power source is switched by switching devices to directly construct an AC waveform. The AC waveform is not electrically isolated from the DC source. As long as the switching elements perform their functions in a symmetrical manner such that the times that the switching elements connect the positive DC voltage and the negative DC voltage to the output is essentially equal, there will be no DC component in the output AC waveform. If the switching elements or their control circuits fail so that switching is no longer symmetrical, however, there can be a DC component in the AC output. This DC component can be detrimental to any utilization equipment connected to the AC converter.
In order to protect these loads, some means of detecting very small DC components in the AC waveform is required. Existing detection circuits using very low-pass filters are limited in accuracy, require expensive tight-tolerance capacitors or inductors, have slow response, reduced accuracy, drift with temperature, and may erroneously respond to low frequency modulation, or transient DC components caused by fault clearing, etc . . . .